The present invention relates to a length or angle measuring device having a graduated-scale support which is mounted to a guide part, wherein the guide part has a coefficient of thermal expansion that differs from that of the graduated-scale support.
Illustrative of such graduated-scale supports are the glass scales of photoelectric length-measuring systems. These glass scales are either (i) applied directly to a metal guide part, usually of aluminum, of a given machine, wherein displacement of a carriage is to be measured, or (ii) they are subassembled in a housing which is also of metal, the subassembly being thereafter securely mounted as a unit to the involved guide.
In such measuring systems, measurement errors occur as a result of the different coefficients of thermal expansion between the scale and the mount or guide part, which errors will be explained by way of example in connection with FIGS. 1 and 2 of the accompanying drawings. In FIGS. 1 and 2, a transparent scale 2 is held in the U-shaped groove of an aluminum mounting strip 1 by a plurality of rubber pieces 3 distributed along its length. Static friction between scale 2 and its mount 1, and the unequal coefficients of thermal expansion of the scale and its mount have the result that, in the circumstance of changing ambient temperature, the scale and its mount become twisted in the plane of grid graduations 4 of scale 2, as shown with exaggeration in FIG. 1. The twisting results in stretching and compressing graduations 4 of the scale, and length changes .DELTA.L of up to 5 .mu.m per meter of scale length can occur. In measuring systems, such as those of coordinate-measurement machines, which typically have a resolution of 0.1 .mu.m, this is clearly not a tolerable change in length.
The indicated length-change effect is particularly disturbing, due to the fact that the scale is not deformed in a reproducible manner but has, rather, a hysteresis behavior due to friction between the scale 2 and its mount 1. It is therefore not feasible to determine the change in length .DELTA.L of the scale graduations with the aid of an additional temperature measurement on the scale and to correct the measurement result of the length-measuring device accordingly.
Furthermore, the scale 2 can also be deformed in the manner shown in FIG. 2, as a result of temperature gradients which build up in the scale itself, and the poor thermal conductivity of the glass precludes effective compensation for such deformations.
Thermal conditions affect not only scales, as in FIGS. 1 and 2, wherein scale graduations are outside the neutral axis and are therefore stretched and compressed upon flexural deformation; but if temperature gradients occur in the scale, the temperature measured by a temperature sensor may possibly not agree with the average temperature which is to be used for an exact correction of the length of the scale. Furthermore, the coefficient of thermal expansion of the inexpensive grades of glass which are frequently used for scales is not known exactly, in that this coefficient can vary in each case, depending on the batch delivered; thus, for this added reason, any subsequent mathematical correction of the length of the scale is only of limited accuracy.
It has already been proposed in West German OS 3,635,511 (U.S. Pat. No. 4,777,728) to mount a scale, without constrictive force, on rollable balls, whereby to reduce non-reproducible scale deformations that are attributable to friction between the scale and its mount. But with this technique, the scale is very strongly insulated thermally from its mount, so that even larger temperature gradients can build up in the scale itself, and between the scale and its mount. Furthermore, with this type of mounting, it is difficult to maintain a uniform distance between the scale and a scale-reading sensor, since weight of the usually relatively thin scale causes the scale to sag between points of ball support.
West German Patent 2,505,587 (British Patent 1,536,365) describes a length-measuring device wherein a measuring scale of glass is mounted to its metallic carrier via an adhesive layer of silicone rubber. It was found, however, that this type of fastening does not reliably avoid deformation of the scale since the elastic adhesive forces of the adhesive layer may still be impermissibly high, particularly in the case of relatively thin scales.
The point made above, as to use of an adhesive layer of silicone rubber, is also true if the scale is mounted to float on a viscous intermediate layer of asphalt, as proposed in West German Patent 1,176,382. And it is additionally noted that this type of mounting can be used only for a horizontal installation of the scale. Furthermore, European Patent 0,266,498 refers to float-mounting of the scale as being prejudicial.
Still further, said German Patent 1,176,382 describes an embodiment in which, as a result of the surface tension of a layer of liquid present between two webs on the mount, the scale is drawn against said webs. But any contact between the scale and the webs of the mount will give rise to frictional forces which result in the aforementioned disadvantages.
The problems described above also arise in angle-measuring devices that rely on a circle of graduations on a glass scale attached to a metal support.